1. Field of the Invention
The present invention relates generally to the aluminum smelting metal from alumina, and more specifically to improvements of the conventional Hall-Heroult aluminum smelting process that include an anode electrode with a sawtooth cross-section which allows oxygen bubbles to be rapidly carried away from the top and liquid aluminum to be drained away from below.
2. Description of the Prior Art
The principle commercial method used for the electrolytic reduction of alumina to aluminum metal is the so-called Hall-Heroult process. This traditional process uses a molten bath of sodium cryolite (Na.sub.3 AlF.sub.6) contained in a carbon-lined cell. Molten aluminum puddles at the bottom of the cell and serves as the cell's cathode. Consumable carbon anodes with flat bottoms are dipped down into the electrolyte bath. Alumina feedstock is introduced to the bath. The alumina dissolves into the electrolyte and is reduced into liquid aluminum droplets when an intense electrical current is passed between the electrodes through the electrolyte.
Typical pot-cell operating temperatures range from 950.degree. C. (1,742.degree. F.) to 1,000.degree. C. (1,832.degree. F.). Oxygen is liberated from the alumina and combines with the carbon in the anodes to produce carbon dioxide gas. Thus the carbon anodes will be consumed and must be periodically adjusted and/or replaced. Large amounts of electricity are also required, which makes aluminum recycling a competitive source of aluminum metal.
On Jun. 3, 1986, U.S. Pat. No. 4,592,812, was issued to Theodore R. Beck, et al., which describes the electrolytic reduction of alumina. A cell used in the reduction has an electrolyte bath of halide salts. A non-consumable anode is positioned at the bottom of the bath, and a dimensionally-stable cathode coated with titanium diboride is spaced above in the bath. Particles of alumina are introduced to the bath and form ions of aluminum and oxygen. The oxygen ions are converted to gaseous oxygen at the anode when electricity is applied. The gaseous oxygen bubbles at the anode and agitates the bath. The aluminum ions are converted to metallic aluminum at the cathode. The cell temperature is just high enough to keep the metallic aluminum molten, and the liquid aluminum accumulates as a pool on top of sludge at the bottom the bath and the secondary cathode.
Theodore R. Beck, et al., were issued U.S. Pat. No. 4,865,701, on Sep. 12, 1989, which describes another electrolytic cell with a bath of halide salts. The anodes and cathodes are vertical plates that are interdigitated and dipped from above into the bath. Bubbling of oxygen at the anodes agitates the bath and resists the settling of alumina particles at the bottom of the bath. Molten aluminum droplets form at the cathodes and flow down to accumulate at the bottom of the bath in a sump.
The use of finely-divided alumina particles in the electrolytic reduction of alumina to aluminum is described by Theodore R. Beck, et al., in U.S. Pat. No. 5,006,209, issued Apr. 9, 1991. Alternating, vertically-disposed cathodes and anodes are used with a horizontally-disposed gas-bubble generator in a molten electrolyte bath of balanced amounts of NaF+AlF.sub.3 eutectic, KF+AlF.sub.3 eutectic and LiF. The gas-bubble generator keeps the alumina particles in suspension. The bath eutectics allow the cell to be operated at a substantially lower temperature, e.g., 660.degree. C. (1220.degree. F.) to 800.degree. C. (1472.degree. F.). The cathodes are made of titanium diboride (TiB.sub.2), a refractory hard metal. The anodes are composed of nickel-iron-copper (Ni--Fe--Cu) cermet. The mean size of the alumina particles introduced to the bath ranges between one micron and one hundred microns, preferably within a range of two to ten microns. The smaller alumina particle sizes are described as being easier to maintain in suspension. But such fine particles are said to have a tendency to agglomerate into clumps which settle out of the bath rapidly. So bottom-located gas generators in the bath are included to deal with this problem.
Theodore R. Beck, et al., describe a non-consumable anode and lining for an aluminum electrolytic reduction cell in U.S. Pat. No. 5,284,562, issued Feb. 8, 1994. The electrolyte used has a eutectic of AlF.sub.3 and either NaF, or primarily NaF with KF and LiF. The anodes used are made of copper, nickel and iron.
A cell for the "production of aluminum with low-temperature fluoride melts" is described, by Theodore R. Beck, in Proceedings of the TMS Light Metals Committee, from the 123.sup.rd TMS Annual Meeting in San Francisco, Calif., Feb. 2, 1994 to Mar. 3, 1994, pp. 417-423, as published by The Minerals, Metals & Materials Society (TMS) 1994. The proposed commercial cell design uses a eutectic electrolyte with a freezing point below 695.degree. C. of either NaF with AlF.sub.3 or a mixture of NaF/AlF.sub.3, KF/AlF.sub.3 and LiF/AlF.sub.3, eutectics operating about 750.degree. C. A five to ten percent slurry, by weight, of Al.sub.2 O.sub.3 with a particle size less than ten microns is required. Close-spaced vertical monopolar anodes and TiB.sub.2 cathodes are used, which makes a pot-room to house a pot-line of such cells dramatically reduced in size over the conventional horizontal-cell pot-rooms.
A horizontal bottom auxiliary anode is used in the cell to agitate the electrolyte to keep sludge from forming from alumina that falls out of suspension, as occurs when the alumina particles agglomerate or are individually larger than ten microns. A device to continuously transport out aluminum produced by the cell is identified as a necessity, but no suitable mechanism is described. Also, feedstocks of alumina with particle sizes less than forty-four microns are generally not available, e.g., because of the severe dust problem such powders can produce. Alumina is injected into the bath from above and contributes to a dust problem due to oxygen capturing alumina dust as it leaves the molten electrolyte surface. In addition, it is difficult in such tall cells to insure that the alumina reaches all the areas of electrolysis. This and the separation of the aluminum from the bottom sludge are problems for the commercial operation with unspecified solutions. Therefore, the description here by Beck of a practical commercial cell is incomplete.
The typical consumable carbon anode used in the Hall-Heroult process operates with a gross cell cathode-anode voltage of about four to five volts. The typical ohmic resistance of the electrolyte is about 0.4 ohms/cm, and a typical current density of 0.75 amps/cm.sup.2 produces a voltage drop of about 0.3 volts/cm. The ohmic drop is thus about 1.2-1.5 volts, the reversible electromotive force (EMF) is about 1.2 volts, the kinetic overpotential is about 0.5 volts, and the gas-bubble layer interface resistance under the anode drops about 0.15 volts. This gives a total of about 3.2 volts that is dropped in the inter-electrode gap. About another volt is lost within the anode and cathode electrodes and their busbar connections. Typically, over thirteen kilowatt hours of electrical energy per kilogram of aluminum metal is needed, and this will result in about 0.45 kilograms of carbon being consumed from the carbon anode.
A typical modern smelting cell will draw about 200,000 amperes and the electrical energy consumed in the pot cells contributes to both the Gibbs energy needed by the chemistry and the ohmic heating that keeps the electrolyte hot. The overall reaction approximates to,
2Al.sub.2 O.sub.3 +3C+energy=4Al+3CO.sub.2.
Alumina has been used as the primary feed material in the electrolytic aluminum smelting metal for over a hundred years. Bauxite, in particular, is the raw material that is universally used. Worldwide, over forty million tons per year of smelting alumina is produced and this, in turn, yields twenty million tons of aluminum metal. The so-called Bayer process is now the principle method used to convert bauxite to alumina, and such process depends on a caustic (e.g., NaOH) to leach the bauxite. Such use of a caustic yields negatively charged alumina. The present inventor, John S. Rendall, has determined that such negatively charged alumina feedstock produced from basic solutions results in a lower solubility and/or rate of dissolution of alumina in the pot cell electrolyte. The negatively charged alumina feedstock slows down the production rate, and causes more electrical power to be needed to drive the electrolysis than would be required if such charges were more positive.
Alumina that is good enough to be used for the electrolytic aluminum smelting is typically referred to as "cell-grade alumina". One of the principle characteristics important to cell-grade alumina is its relative solubility in molten fluoride salt electrolyte. This is the reason that molten fluoride salt electrolytes are heated to 950.degree. C. Higher temperatures will raise the solubility. But even at these higher temperatures, the best solubility obtained is only about four percent by weight. An alumina that is inherently more soluble would allow better percentages at lower temperatures.
The optimal alumina reactivity and the optimum electric voltage needed to produce a useful electrolytic dissociation of the alumina has been the subject of a great deal of scientific study. Just about all electrolytic cells are engineered to use alumina that has been precipitated as a hydroxide from caustic solutions, e.g., with nine pH.
About four percent alumina, Al.sub.2 O.sub.3, by weight, is considered the upper limit of solubility at 950.degree. C. The usual way that alumina is fed into pot cells produces a lot of dust. Such alumina feed is also used in fluid beds to capture fluoride emissions. The voltage drop of four to six volts across a conventional electrolytic cell includes the bath resistance, the electrode resistance of two electrodes, and the energy of electrolysis ameliorated by the electrolytic formation of CO.sub.2.
The solubility of the alumina in the molten bath is so low, it requires careful and sophisticated replenishment. Over-feeding the alumina can create a bottom sludge that can cover and electrically isolate the molten aluminum cathode surface. This will cause a reduction in the electrical current that can be induced due to the increased voltage required, and thereby cause the cell to freeze up because it cannot produce enough electrical heating.
When the alumina in solution drops under one percent, by weight, an increase in the voltage drop occurs in the carbon anode. This, in turn, reduces the power input and heat generated, again causing the cell to freeze. Any localized heating can adversely affect the solid crust that forms at the top of the pot cell, and carbon fluoride gases can be released.